1. Field of the Invention
The present invention relates to binder compositions for preparing solvent-free coating compositions and to their use as coatings for protecting metallic and mineral substrates against corrosion, erosion and mechanical damage.
2. Description of the Prior Art
Coatings to protect substrates against damage such as corrosion, erosion or mechanical damage caused by external influences are known and described, for example, in Glasurit-Handbuch, Lacke und Farben, 11th Edition, p.494 ff., Curt R. Vincentz Verlag, Hanover, 1984 and DIN 55 928, Parts 5 and 9.
These coating compositions may be divided into one-component systems (1C), two-component epoxy resin systems (2C-EP) and two-component polyurethane systems (2C-PUR). Although physically drying 1C coating compositions (e.g., vinyl chloride copolymers or acrylate copolymers) have good resistance to aqueous salt solutions, acids and bases, they have poor resistance to animal or vegetable fats and greases, solvents and chemicals. In addition, these copolymers require large amounts of organic solvents to formulate binder mixtures having viscosities that meet practical requirements.
Oxidatively drying 1C coating compositions (e.g, alkyd resins and epoxy resin esters) are characterized by good substrate adhesion and improved resistance to solvents and chemicals, but deteriorate when attacked by acids or bases. The binder compositions also require organic solvents when preparing formulations to meet practical requirements.
Coatings based on 2C-EP combine good mechanical strength with good resistance to solvents, chemicals, acids and bases, and are additionally characterized by very good substrate adhesion. The binder compositions can be formulated as low-solvent, solvent-free or aqueous compositions. A significant disadvantage of the resulting coatings is their known poor elasticity, especially at low temperatures (see for example Kunststoff-Handbuch, Vol. 7; Polyurethanes, 2nd Edition, Editor G. Oertel, Hanser Verlag, Munich, Vienna, 1983, pp. 556-8). This brittleness leads to poor crack coverage by the coating, with the result that the substrate can be attacked.
A balanced combination of hardness and elasticity is a valuable property of 2C-PUR coatings and a major advantage when compared to 2C-EP coatings. Although the resistance of 2C-PUR coatings to chemicals, solvents, acids and bases is just as good as that of 2C-EP systems, the substrate adhesion is generally worse.
The hydroxy-functional binder component of 2C-EP coating compositions can be based on various types of chemical structures. For example, 2C-PUR coatings based on polyacrylate polyols have good light and weathering resistance and are also resistant to saponification, solvents and chemicals. A disadvantage is the relatively high viscosity of the binder compositions, which often requires the use of organic solvents.
2C-PUR binder compositions based on polyester polyols have significantly lower viscosities and, thus, can be formulated to be solvent-free. Although they exhibit good resistance to chemicals, solvents, weathering and light, these coatings have the disadvantage of poor resistance to saponification, which severely restricts their use for corrosion protection of metallic substrates and also for coating mineral (alkaline) substrates.
Hydroxy-functional polyester polyacrylate binders for lightfast, solvent-free coating compositions are described in EP-A-580,054. These products combine the good resistance properties of the polyacrylates with the low viscosities of the polyesters, but require relatively large amounts of hydrophobic reactive thinners to achieve a sufficiently low viscosity as well as cyclic esters to extend the hydroxy side groups of the polyacrylate chains with hydroxyester units.
2C-PUR coatings based on polyether polyols exhibit significantly better resistance to saponification as well as good resistance to solvents and chemicals. Also, the binder compositions due to their low viscosity can be formulated to be low-solvent or solvent-free. However, these coatings have the disadvantage of poor substrate adhesion and poor resistance to weathering and light.
An object of the present invention is to provide a solvent-free, low viscosity coating composition that does not exhibit the disadvantages of the prior art and can be used to prepare coatings for metallic substrates or mineral substrates having good resistance to corrosion, erosion and mechanical damage.
It has now surprisingly been found that this object can be achieved with a binder composition based on a polyisocyanate and a polyether polyacrylate.